1. Field of the Invention
The invention disclosed in the present specification relates to a magnetic recording medium. The invention disclosed in the present specification particularly relates to a magnetic recording medium for use in energy-assisted magnetic recording methods. The invention disclosed in the present specification more particularly relates to a magnetic recording medium for use in thermally-assisted magnetic recording methods.
2. Background of the Related Art
Perpendicular magnetic recording methods are used as a technique for realizing higher magnetic recording densities. A perpendicular magnetic recording medium contains at least a nonmagnetic substrate and a magnetic recording layer formed from a hard magnetic material. A perpendicular magnetic recording medium may also optionally contain, for example, a soft magnetic underlayer that is formed from a soft magnetic material and functions to concentrate, in the magnetic recording layer, the magnetic flux produced by the magnetic head; a seed layer for inducing orientation, in a desired direction, of the hard magnetic material of the magnetic recording layer; and a protective film that protects the surface of the magnetic recording layer.
Reducing the grain size of the magnetic crystal grains in magnetic recording layers has become a pressing requirement in recent years in order to bring about additional improvements in the recording density of perpendicular magnetic recording media. On the other hand, reducing the grain size of magnetic crystal grains causes a reduction in the thermal stability of the written magnetization (signal). As a consequence, in order to compensate for the reduction in thermal stability caused by reducing the grain size of the magnetic crystal grains, the formation is required of magnetic crystal grains using materials that have higher magnetocrystalline anisotropies.
L10-type ordered alloys have been proposed as materials that have the required high magnetocrystalline anisotropies. WO 2013/140469 (Patent Literature 1) describes an L10-type ordered alloy containing at least one element selected from the group consisting of Fe, Co, and Ni and at least one element selected from the group consisting of Pt, Pd, Au, and Ir. Representative L10-type ordered alloys include, for example, FePt, CoPt, FePd, and CoPd.
However, magnetic recording media that have a magnetic recording layer formed of a highly magnetically anisotropic material exhibit a high coercivity and the writing of magnetization (signal) is then made more difficult. Energy-assisted magnetic recording methods, e.g., thermally-assisted recording methods and microwave-assisted recording methods, have been introduced in order to overcome this difficulty in writing. Thermally-assisted recording methods utilize the temperature dependence of the magnetic anisotropy constant (Ku) of magnetic materials, i.e., the characteristic that Ku declines as the temperature increases. These methods use a head that has a heating function for the magnetic recording layer. Thus, by temporarily lowering Ku by raising the temperature of the magnetic recording layer, the reversal magnetic field can be lowered and writing can be carried out at this point. Since Ku returns to its original high value upon cooling, a stable written signal (magnetization) can be maintained. WO 2013/140469 proposes a method that facilitates thermally-assisted magnetic recording by establishing a large temperature gradient in the in-plane direction of the magnetic recording layer during writing.
The use of a thermally-assisted recording method requires that a means of heating the magnetic recording layer be disposed in the magnetic head used for writing. Due, however, to the various requirements imposed by the magnetic head, there are limitations on the heating means that can be used. When this point is considered, the heating temperature of the magnetic recording layer during writing is desirably as low as possible. One index for the heating temperature is the Curie temperature Tc. The Curie temperature Tc of a magnetic material refers to the temperature at which the material loses its magnetism. By lowering the Curie temperature Tc of the material of the magnetic recording layer, the magnetic anisotropy constant Ku at a given temperature is lowered and writing is then made possible at a lower heating temperature.
However, a strong correlation exists between the Curie temperature Tc of a magnetic material and its magnetic anisotropy constant Ku. Materials that have a large magnetic anisotropy constant Ku generally have a high Curie temperature Tc. Due to this, a reduction in the magnetic anisotropy constant Ku and a lowering of the Curie temperature Tc have heretofore been carried out where the priority has been to lower the heating temperature. In relation to this problem, Japanese Patent Application Laid-open No. 2009-059461 (Patent Literature 2) proposes that the correlation between Ku and Tc be relaxed by disposing a plurality of magnetic layers and by establishing a different Ku and Tc for each magnetic layer. Specifically, this literature proposes a magnetic recording layer that comprises a first layer having a first Curie temperature Tc1 and a second layer having a second Curie temperature Tc2 wherein Tc1 is higher than Tc2. By heating this magnetic recording layer to a temperature equal to or greater than Tc2, exchange coupling between the first layer and the second layer is extinguished and writing magnetization to the first layer is made possible.
There have also been efforts to introduce various additional elements into L10-type ordered alloys with the goal of improving various other properties. For example, Japanese Patent Application Laid-open No. 2003-313659 (Patent Literature 3) proposes a sintered sputtering target that contains the elements constituting an L10-type ordered alloy and that also contains additional elements and has an oxygen content of not more than 1000 ppm. It is taught that the thin film formed using this target can achieve the ordering of an L10-type ordered alloy at lower annealing temperatures. In particular, an even greater promotion of the ordering of the L10-type ordered alloy is brought about when, for example, Cu or Au is added. Japanese Patent Application Laid-open No. 2003-313659 also discloses that isolation of L10-structured magnetic crystal grains by a nonmagnetic material contributes to improving the magnetic recording density. Nonmagnetic elements and nonmagnetic compounds disposed around the magnetic crystal grains for the purpose of bringing about magnetic isolation between magnetic crystal grains are listed. Various materials containing Ru are taught as examples of such materials.
U.S. Patent Application Publication No. 2003/0162055 (Patent Literature 4), on the other hand, provides a magnetic recording layer formed from a polycrystalline ordered alloy that has the composition (CoX)3Pt or (CoX)3PtY and an ordered structure different from that of the L10 type. Here, the additional element X migrates to the grain boundary and has the effect of promoting magnetic isolation between the magnetic crystal grains. The additional element Y has the effect of facilitating control of the magnetic properties of the polycrystalline ordered alloy, the distribution of the magnetic crystal grains, and the magnetic isolation. US Patent Application Publication No. 2003/0162055 describes various materials that contain Ru as examples of the additional element X.
However, the current situation is that there has been little progress in research on Ru as a material added to ordered alloys. There has been little progress in research on the magnetic characteristics of ordered alloys to which Ru has been added and in particular on the relationship in such ordered alloys between the magnetic anisotropy constant Ku and the Curie temperature Tc.
The objects of the invention disclosed in the present specification are to provide a magnetic recording layer that has a high magnetic anisotropy constant Ku and a low Curie temperature Tc and to provide a magnetic recording medium that has this magnetic recording layer.